Negative electron affinity based photocathodes, often composed of group III-V elements, are used in many applications. In technological constructs, they are frequently employed as sensitive generators of photoelectrons fed into a cascade chain for signal amplification in photomultiplier tubes. Specialized negative electron affinity photocathode based tubes are used for low level light amplification in night-vision goggles and sights. Scientific applications include use as sources of spin-polarized, high intensity and/or duty cycle and ultra-cold electrons. In all cases, the photocathode is activated to lower its electron affinity, thereby enabling photoelectrons to be emitted via excitation by relatively low energy visible and near infrared photons.
Activated photocathodes are restricted to operation in the very best ultra-high vacuum environments so that they exhibit stable operation over long periods. In sealed tubes, exposure is limited to gas generated from internal components through electron bombardment and heating. When used as a bolt-on electron source in an open system, the gas load may be compounded by connection to vacuum systems with higher pressures than the source. A major problem in the preparation and use of these photocathodes is the relatively high chemical reactivity of both the clean and activated III-V photocathode surfaces. Recently, there has been an advance in activation process methodology to enhance the chemical immunity of activated III-V photocathodes. Regardless of the activation method used, all photoemitters exhibit loss of photoyield with time due to interaction with the background gas. In open systems, this can be particularly egregious. To maintain the photoemitter's photoyield at a usable level, the standard, conventional method is to apply additional cesium to the surface, thereby partly restoring the photoyield to a substantial fraction of the maximum achieved during the activation process. The availability and/or presence of free cesium in an operational environment may be mechanically difficult to achieve or result in high voltage breakdown. Prior to this invention, no satisfactory alternate method had been developed so that GaAs and other III-V based photoemitters could have their photoyield recovered in an alkali application-free fashion.
GaAs photoemitters are activated to the negative electron affinity state by first starting with an atomically clean surface. Such a surface is obtained by chemical treatment, frequently followed by heating once the photoemitter has been introduced into a vacuum environment. Activation consists of the deposition of a low work function metal, such as a group IA alkali, followed or interleaved with an oxidizing agent onto the clean surface. The lowest affinities are obtained using cesium as the alkali and either oxygen or nitrogen trifluoride as the oxidizer. Studies of the photoyield decay process have shown that the oxidizer absorption sites' numbers and initial stoichiometry can change with further gas exposure causing the photoyield to decrease. Low energy electrons are well understood to possess the correct reactivity to induce electron stimulated desorption. Together these facts suggest that if a diminished photoyield photocathode were exposed to a low energy electron beam, the subtle surface chemical reactions that led to the diminished photoyield may be reversed.
An alkali application-free recovery and resurrection method would have great utility in many applications of lowered and negative electron affinity based photocathodes. Decreased alkali introduction into high electric field structures would enhance the shelf- and operational-lifetimes of these devices by reducing the probability of field emission induced breakdown. Photocathodes in sealed systems lacking an alkali source could be rejuvenated using this new methodology by employing the photocathode as its own electron source and impinging the photogenerated electrons back upon the photocathode's surface via reflection or redirection by application of pulsed voltages.